A Reactive Manganese(IV)–Hydroxide Complex: A Missing Intermediate in Hydrogen Atom Transfer by High-Valent Metal-Oxo Porphyrinoid Compounds

High-valent metal-hydroxide species are invoked as critical intermediates in both catalytic, metal-mediated O 2 activation (e.g., by Fe porphyrin in Cytochrome P450) and O 2 production (e.g., by the Mn cluster in Photosystem II). However, well-characterized mononuclear M IV (OH) complexes remain a rarity. Herein we describe the synthesis of Mn IV (OH)(ttppc) ( 3 ) (ttppc = tris(2,4,6-triphenylphenyl) corrole), which has been characterized by X-ray diffraction (XRD). The large steric encumbrance of the ttppc ligand allowed for isolation of 3 . The complexes Mn V (O)(ttppc) ( 4 ) and Mn III (H 2 O)(ttppc) ( 1 ·H 2 O) were also synthesized and structurally characterized, providing a series of Mn complexes related only by the transfer of hydrogen atoms. Both 3 and 4 abstract an H atom from the O–H bond of 2,4-di- tert -butylphenol (2,4-DTBP) to give a radical coupling product in good yield ( 3 = 90(2)%, 4 = 91(5)%). Complex 3 reacts with 2,4-DTBP with a rate constant of k 2 = 2.73(12) × 10 4 M –1 s –1, which is ∼3 orders of magnitude larger than 4 ( k 2 = 17.4(1) M –1 s –1 ). Reaction of 3 with a series of para -substituted 2,6-di- tert -butylphenol derivatives (4-X-2,6-DTBP; X = OMe, Me, t Bu, H) gives rate constants in the range k 2 = 510(10)–36(1.4) M –1 s –1 and led to Hammett and Marcus plot correlations. Together with kinetic isotope effect measurements, it is concluded that O–H cleavage occurs by a concerted H atom transfer (HAT) mechanism and that the Mn IV (OH) complex is a much more powerful H atom abstractor than the higher-valent Mn V (O) complex, or even some Fe IV (O) complexes.